This invention is in the field of construction and more specifically, the construction of concrete walls used in housing animals. The use of concrete slabs in parallel spaced relationship forming a slotted floor to support animals is well known. Such a floor is disclosed, for example, in my U.S. Pat. Nos. 4,119,691 and 4,168,820. The floors may be precast at a production facility and then transported to the use site or may be poured on the use site thereby saving the transportation costs encountered in the former method. The vertical walls of the animal enclosure may be poured on site using conventional concrete forms. In the event vertically extending ventilation slots are desired in the side walls, then it is necessary to produce the walls at a separate production facility utilizing expensive and hard-to-transport molds. Heretofore, such slotted side walls have either been precast and transported to the use site or metal fence-like walls have been utilized. Disclosed herein is a thin-walled lightweight mold to be used with conventional concrete forms for producing a vertically standing slotted concrete wall poured at the use site in final position. The molds may be used with additional pairs of forms to integrally connect the slotted concrete wall with an unslotted concrete wall.